The present invention relates to the offset lithographic printing blankets, and more particularly, to tubular offset lithographic printing blankets and methods for manufacturing the same.
A web offset printing press typically includes a plate cylinder, a blanket cylinder and an impression cylinder supported for rotation in the press. The plate cylinder carries a printing plate having a rigid surface defining an image to be printed. The blanket cylinder carries a printing blanket having a flexible surface which contacts the printing plate at a nip between the plate cylinder and the blanket cylinder. A web to be printed moves through a nip between the blanket cylinder and the impression cylinder. Ink is applied to the surface of the printing plate on the plate cylinder. An inked image is picked up by the printing blanket at the nip between the blanket cylinder and the plate cylinder, and is transferred from the printing blanket to the web at the nip between the blanket cylinder and the impression cylinder. The impression cylinder can be another blanket cylinder for printing on the opposite side of the web.
A conventional printing blanket is manufactured as a flat sheet. Such a printing blanket is mounted on a blanket cylinder by wrapping the sheet around the blanket cylinder and by attaching the opposite ends of the sheet to the blanket cylinder in an axially extending gap in the blanket cylinder. The adjoining opposite ends of the sheet define a gap extending axially along the length of the printing blanket. The gap moves through the nip between the blanket cylinder and the plate cylinder, and also moves through the nip between the blanket cylinder and the impression cylinder, each time the blanket cylinder rotates.
When the leading and trailing edges of the gap at the printing blanket move through the nip between the blanket cylinder and an adjacent cylinder, pressure between the blanket cylinder and the adjacent cylinder is relieved and established, respectively. The repeated relieving and establishing of pressure at the gap causes vibrations and shock loads in the cylinders and throughout the printing press. Such vibrations and shock loads detrimentally affect print quality. For example, at the time that the gap relieves and establishes pressure at the nip between the blanket cylinder and the plate cylinder, printing may be taking place on the web moving through the nip between the blanket cylinder and the impression cylinder. Any movement of the blanket cylinder or the printing blanket caused by the relieving and establishing of pressure at that time can smear the image which is transferred from the printing blanket to the web. Likewise, when the gap in the printing blanket moves through the nip between the blanket cylinder and the impression cylinder, an image being picked up from the printing plate by the printing blanket at the other nip can be smeared. The result of the vibrations and shock loads caused by the gap in the printing blanket has been an undesirably low limit to the speed at which printing presses can be run with acceptable print quality.
In response to these deficiencies in conventional flat printing blankets, gapless tubular printing blankets were developed by the assignee of the present invention. These gapless tubular printing blankets are described, for example, in U.S. Pat. Nos. 5,768,990, 5,553,541, 5,440,981, 5,429,048, 5,323,702, and 5,304,267.
In this regard, U.S. Pat. No. 5,304,267 is directed to a method of manufacturing a gapless tubular printing blanket. The specification of this patent describes a preferred method of manufacturing a gapless tubular printing blanket as xe2x80x9ccoating a compressible thread with a mixture of rubber cement and microspheres, and wrapping the coated thread in a helix around the cylindrical sleevexe2x80x9d to form a compressible layer; xe2x80x9ccoating an inextensible thread with a rubber cement that does not contain microspheres, and wrapping the coated thread in a helix around the underlying compressible layerxe2x80x9d to form an inextensible layer, and xe2x80x9cwrapping an unvulcanized elastomer over the inextensible layer, securing it with tapexe2x80x9d and vulcanizing xe2x80x9cthe taped structure . . . so that a continuous seamless tubular form is taken by the overlying layers of elastomeric material.xe2x80x9d Additional methods of manufacture are also described, including the manufacture of a gapless tubular printing blanket having a circumferentially inextensible sublayer comprising a continuous piece of plastic film extending in a spiral through the elastomeric material of an inextensible layer and around a compressible layer. The plastic film preferably has a width approximately equal to the length of the tubular printing blanket, and a thickness of only 0.001 inches so that the narrow scam defined by the 0.001 inch wide edge of the uppermost layer thereof will not disrupt the smooth, continuous cylindrical contour of an overlying printing layer.
DE 197 20 549 A1 purports to describe a method for manufacturing a cylinder carrier by winding of a continuous strip onto a supporting mandrel surface. The strip is unwound from a spool which is mounted so that it can pivot so that the strip winding angle is self adjustable. Strip tension is maintained during the winding process. Preliminary conditioning treatment and coating of the strip with an adhesive takes place between unwinding and winding of the strip. The preliminary treatment stations are mounted on a support wall which is installed so that it can pivot relative to the cylinder surface. The cylindrical carrier shell is coated with an integral layer of plastic material. The carrier shell is shown as having a fixed length.
The methods for manufacturing gapless tubular printing blankets described above suffer from the deficiency that they produce blankets in batch mode (i.e. one at a time) with a fixed axial length. Batch mode production increases production costs, increases production time, and results in batch to batch variability in the blankets produced.
Commonly-assigned U.S. Pat. No. 6,257,140, filed Dec. 27, 1999 and which is hereby incorporated by reference herein, describes gapless tubular printing blankets produced continuously and cut to length as desired. The sleeve and print layer are xe2x80x9ccontinuouslyxe2x80x9d formed in that the sleeve forming station continues to form an additional portion of the sleeve while the print layer forming station applies the print layer to the previously formed portion of the sleeve. Wound tapes or cross-head extruders are used to apply various layers.
The present invention provides for ribbon casting of materials to form various layers of a tubular printing blanket. xe2x80x9cRibbon castingxe2x80x9d as defined herein can mean that a liquid material is deposited from a stationary source onto a rotating and translating substrate or that a liquid is deposited from a rotating source onto a translating substrate. A continuous ribbon of liquid material thus can be placed on the substrate.
xe2x80x9cLiquid materialxe2x80x9d as defined herein can be any flowable material, including a semi-solid material. The liquid material preferably is a polymer which does not require a separate curing step, i.e. a self-cure material. Since the liquid can be sent out from a single orifice of the source, the depositing of the material to form the blanket is simpler than a cross-head extruder, in which material is forced out so as to contact the entire circumference of the substrate. Moreover, liquid materials are simpler to use than tape materials.
The present invention thus provides a device for manufacturing a tubular printing blanket comprising:
a sleeve translation device for moving a sleeve, the sleeve providing an innermost support layer for the printing blanket; and
at least one ribbon casting device applying a flowable material, the flowable material forming a layer disposed over the sleeve.
The present device provides for more cost-effective and quicker manufacture of printing blankets. The cost of tubular blankets is a large factor in the overall costs of operating a printing press using tubular printing blankets.
The at least one ribbon casting device preferably includes a compressible layer ribbon casting device having a first supply area for a flowable material and a compressibility forming device, which can for example be a supply area for compressible microspheres or an air blower or foamer. The foam structure or the microspheres can provide the compressibility desirable for the compressible layer.
The at least one ribbon casting device also preferably includes a reinforcing layer ribbon casting device and a print layer ribbon casting device.
The ribbon casting devices preferably have a single nozzle through which the flowable material flows onto the respective substrate.
Preferably, the ribbon casting devices are stationary, and the sleeve translating device is a translating and rotating device, on which a continuous sleeve is being formed, for example using a metal tape.
Alternatively, the ribbon casting devices may rotate in a circular motion about the substrate and the sleeve translating device may continuously translate a sleeve substrate past the ribbon casting devices.
The present invention provides a method for forming a tubular printing blanket comprising the steps of:
translating a sleeve in a first direction; and
ribbon casting at least one of a compressible layer, reinforcing layer and a print layer about the sleeve as the sleeve translates.
Preferably, the method further includes rotating the sleeve during the translating step. The ribbon casting step preferably includes ribbon casting a compressible layer, a reinforcing layer and a print layer.
Alternately, the ribbon casting step may include rotating a ribbon casting device about the sleeve as the sleeve translates.
While rubber could be used for ribbon casting, the rubber then typically is cured in a separate step. It is highly advantageous that a polymer which does not need a separate curing step be used in the ribbon casting process, such a polymer being defined herein as a xe2x80x9cself-cure polymerxe2x80x9d. Most preferably, urethane is used to form blankets according to the present invention. Urethane has the advantages of flowing well during a ribbon casting and of setting quickly. However, the self-cure polymer also could be a self-vulcanizing rubber such as RTV (room temperature vulcanizing) rubber.
The ribbon casting step thus preferably includes ribbon casting urethane to form the at least one layer.
Preferably, the blankets are formed continuously so as to have an indeterminate length. The method then further includes the step of cutting the sleeve to a desired length so as to form the blanket.
The present invention also provides a tubular printing blanket comprising a sleeve, a compressible layer and a print layer, at least one of the compressible layer and the print layer being made of urethane.
Preferably, both the compressible layer and the print layer are made of urethane, and a reinforcing layer is located between the compressible layer and the print layer. The reinforcing layer is also preferably made of urethane.
Preferably, the compressible layer is made of urethane foam formed by blowing carbon dioxide, air or another blowing agent into the urethane before it exits the nozzle of a ribbon casting device. Compressible microspheres however could also be embedded in the urethane to provide the compressibility.
The reinforcing layer preferably is made of a high durometer urethane of greater than 80 shore A, most preferably about 100 shore A. The reinforcing layer preferably is thinner than the compressible layer.
The print layer preferably is made of a urethane with a durometer of less than 80 shore A and most preferably of about 60 shore A.
Similar durometer values can be provided for blankets according to the present invention made with self-cure polymers other than urethane.
The layers preferably are provided by ribbon casting which forms a spiral shape which melds together to form uniform gapless layers.
The sleeve preferably is made of steel, preferably formed by a ribbon in a continuous fashion.